Diaphragms which are produced from paper made from wood pulp are often used in loud speakers. In the process of manufacturing a diaphragm for a loud speaker, a beating operation is followed by a paper making operation, in general. During the beating operation, external mechanical forces are applied to shear, compress and crush bundles of fibers by a beater.
The objectives of the beating operation are to soften the fibers by splitting the fibers into fibrils, that is, minute fibers like root hair and to cause the fibers to become firmly entangled together in the subsequent paper making operation. This operation imparts good physical properties such as tear strength, tensile strength and elasticity to a paper diaphragm.
As shown in FIG. 4(a), a wood pulp fiber 60 has an outer layer as secondary walls to hold fibrils inside of the walls. The outer layer structure is finer than the inner structure of the wood pulp fiber 60.
Mechanical forces are applied while making the blade of the beater contact with the wood pulp fiber 60, and the resultant fiber 70 is shown in FIG. 4(b). Fibrils 70a are exposed at the cut sections on both ends of the fiber 60 and at a damaged section of the outer layer. However, the exposed fibrils 70a are short and the outer layer is not crushed sufficiently.
In a conventional method for manufacturing a diaphragm for a loud speaker, external mechanical forces are repeatedly applied to the fibers 70. Therefore, at the time the fibers 70 are split into the fibrils 70a, the fibers 70 are cut up and the fiber tissues are destroyed, resulting in deterioration of the physical properties of paper. Additionally, since only a small amount of the fibrils 70a are exposed, the fibers 70 do not get entangled together in paper making. Furthermore, it is difficult to expose a substantially uniform amount of fibrils 70a every time the wood pulp fibers 60 are beaten.
Thus, in the conventional manufacturing method, it is complicated to control the beating operation and difficult to manufacture diaphragms with uniform quality. And this causes the quality of loud speakers to become inconsistent.